Thermal proximity imaging or sensing is a technique by which topographical variations (e.g., asperities, projections, recesses, etc.) on a surface of a medium can be detected and characterized by monitoring the thermal response of a sensor in motion relative to the surface. As disclosed in the above-identified U.S. Pat. No. 5,527,110, a sensor, possibly a magnetoresistive access element used to access data on a data storage medium, is heated using a known bias current. The Joule effect heat induced in the sensor by the bias current varies when topographical variations on the surface pass by the sensor during the relative movement between the sensor and the surface. Because the topographical variations vary the distance ("height" or "gap") between the sensor and the surface, and because the heat transferred from the sensor varies as a function of this distance, measuring or monitoring the temperature change of the sensor is a useful technique for identifying the location and character of the topographical variations on the medium.
Thermal proximity sensing ("TPS") is a very sensitive and reliable indicator of disk surface topography. It can serve as an imaging tool, defect detector and a sensitive indicator of head dynamics. Thermal proximity sensing has been shown to be a useful tool for "in situ" measurement of disk topography and for defect detection. It is a non-invasive technique using the variation of temperature of the sensor element with gap spacing. Sensitivity is in the sub-nanometer range for height, and thermal response is in the MHz range for current head geometries and bias current arrangements. However, thermal proxirnity sensing is not an intrinsically self-calibrating method and, therefore, for certain demanding applications, a technique for providing calibrated, quantitative height information is an important requirement.
For qualitative applications such as visually inspecting the medium surface for topographical features, reliance on an uncalibrated TPS image is often sufficient. However, for more demanding applications such as thermal glide screening for large defects, calibration of the thermal response to the distance between the sensor and the surface is important. Unlike magnetic measurements of head-disk spacing in which the signal amplitude varies exponentially with height and recorded wavelength (in accordance with the Wallace Spacing Law), TPS responses are head-dependent and require calibration. It has been observed that thermal responses can vary by a factor of two or more for a sampling of nominally identical suspension-mounted heads. Thus, calibration is required to make estimates of height with greater accuracy than this factor of two.
Therefore, to provide quantitatively accurate images of topographical variations on a surface over which a TPS sensor is to be moved, techniques for calibrating the sensor are required. The calibration techniques should support sensors designed for non-invasive, in-situ measurement of topography in storage systems, and should therefore, preferably, not require any particular, non-standard use or manipulation of the surface itself.